 Alpha particles may be ejected from the nucleus of an atom during radioactive decay. They are relatively heavy and travel only about an inch in air. They can also be easily shielded by a single sheet of paper, for example, and cannot even penetrate the outer dead layer of skin. So they pose no danger when their source is outside the human body. Beta particles are essentially electrons emitted from the nucleus of a radioactive atom. They are lighter than alpha particles and can travel farther in air up to several yards. Very energetic beta particles can penetrate up to half an inch through skin and into the body. They can be shielded with less than an inch of material, such as plastic. In the case of lower energy beta particles, the outer layer of clothing can act as an effective shield. Gamma rays, the most energetic part of the electromagnetic spectrum, can be emitted from the nucleus during radioactive decay. They are able to travel tens of yards or more in air and easily penetrate the human body. Shielding this penetrating type of radiation requires thick, dense material, such as several inches of lead or concrete. Neutrons can be released from the nucleus of an atom during a fission reaction, such as within a nuclear reactor or upon detonation of a nuclear weapon. Neutrons, like gamma rays, are very penetrating and several feet of concrete is needed to shield against them.